1. Field of the Invention
This invention relates to telephone equipment, and more particularly to hold circuitry for use with telephones using tone actuated dialing systems.
2. Prior Art
While nominally controlled by the subscriber, connecting and disconnecting telephone circuits is actually accomplished at the telephone company central office in response to the conditions appearing on the line. In the case of a disconnection, central office equipment monitors the load on the line presented by the originating subscriber's telephone, and disconnects the circuit shortly after the load disappears. In order to allow a subscriber to hang up one telephone and later pick up on the same or another instrument, a "hold" feature is provided on some systems. The hold function is normally accomplished by connecting a resistor across the line to simulate an instrument load, after which the phone can be put on-hook without a disconnect. There are several methods in the prior art for mechanizing such a hold function.
In one system, a momentary button switch on the instrument energizes an electrically latched relay which in turn connects the required resistor across the line. The phone may then be put on-hook without a disconnect. When any phone on the system is subsequently taken off hook, the latched relay is deenergized and the resistor thus disconnected and the hold removed.
A second system for connecting and disconnecting the hold resistor utilizes a silicon controlled rectifier in series with the hold resistor. The silicon controlled rectifier is switched on by a momentary button switch on the phone, effectively connecting the hold resistor across the line. Hold is cancelled by switching the silicon controlled rectifier off in response to the reduced line voltage which occurs when a phone is taken off-hook.
The prior art systems have several disadvantages. For example, they either require substantial extra wiring or they require extra equipment at each telephone on the line. They may also require access to a source of power, either batteries or a power line. A further disadvantage is that a special button for the hold function must be installed if ordinary single line telephones are to be used. Such buttons are combersome, detract from the aesthetics of the installation and present extra service problems.
The present invention does not suffer from these disadvantages. An ordinary single line instrument, without modification, may be used with the invented system; only one simple circuit is used to service a line irrespective of the number of telephones in use on the line; and the hold circuit requires no external power. The invented hold circuit can be made to be plugged into any unused jack on the line.